The quantitative analyses of analytes such as glucose, cholesterol and lactic acid (or lactate) in a whole blood sample give useful information for diagnosis and treatment of various deceases. In the analyses of these analytes, a dry analytical method utilizing a oxidase reaction system which produces hydrogen peroxide is generally employed. The dry analytical method is performed using an analytical element in the form of a film or a sheet which contains one or more reagents which react with an analyte to show coloring in the element. The dry analytical element is generally held in a mount.
The dry analytical method is advantageous over a wet analytical method from the view points of handling and rapidity in the analytical procedures. However, if a whole blood per se is used as a sample solution for the analysis, the use of a dry analytical element encounters a problem due to the presence of hematocytes (i.e., blood corpuscles) and other solid polymeric components in the blood. In more detail, when the whole blood is spotted on the analytical element, the hematocytes and other solid components are plugged in the element, and there fore a certain portion of the spotted whole blood is not accepted by the element to lower accuracy of the analysis. Further, the plugged hematocytes adversely influence the colorimetry, that is, coloration of hematocytes disturbs measurement of coloring caused by the analytical reaction. In order to solve these problems, hematocytes and other solid components are removed in advance of the analysis, or a spreading layer capable of receiving the hematocytes therein is arranged on the analytical element in the form of an outermost layer.
Japanese Patent Provisional Publication No. 60-82859 discloses a multilayer analytical element comprising a transparent (i.e., light-transmissive) support, an indicator layer containing a dye-forming composition and peroxidase, an oxidase-containing layer, an oxygen-permeable, protein-impermeable, light-shielding layer, and a porous spreading layer, in order. In the analytical element, the dye-forming composition is a composition of 4-aminoantipyrine and its coupler (i.e., 1,7-dihydroxy-naphthalene), and the porous spreading layer is made of 100% cotton broadcloth. The light-shielding layer shields red color of hemoglobin from the coloring produced by the analytical reaction and further functions as a light-reflecting layer. The analytical element disclosed in the publication has an oxidase-containing layer or a mordant layer containing oxidase between the indicator layer and the light-shielding layer. This arrangement enables to obviate lowering of analytical accuracy caused by hydrogen peroxidase-decomposing components in the whole blood, such as catalase. It is known that oxidase functions satisfactorily under enough supply of airy oxygen. Therefore, the desired rapid analysis cannot be expected because the oxidase-containing layer cannot receive enough airy oxygen supply due to the presence of the light-shielding layer and spreading layer placed on the oxidase-containing layer.
Japanese Patent Publication No. 60-111960 discloses a multilayer analytical element for the analysis of whole blood which has no filter layer, that is no spreading layer. The analytical element has, on a light-transmissive support sheet, a recording zone comprising gelatin and a reagent-coated zone in order. The reagent-coated zone is a coated layer comprising beads, oxidase, peroxidase, and a color-forming composition of 4-aminoantipyrine and its coupler. The reagent-coated zone of the analytical element shows high light-shielding power. In the analytical procedures, a dye produced in the reagentcoated zone is transferred into the recording zone (i.e., gelatin layer) and then measured by colorimetry at a wavelength of 600 nm or longer. A color produced by the reaction of 4-aminoantipyrine and its coupler varies depending upon the selection of coupler. For instance, the use of 8-anilino-l-naphthalenesulfonic acid as the coupler gives a dye having a absorption area at 600 nm or longer wavelength. The calorimetric measurement at a wavelength of 600 nm or longer enable to remove the adverse effect of the colored solid components in the whole blood. However, the transfer of the produced dye is accompanied by diffusion of the dye, so that the analytical accuracy lowers.
Japanese Patent Provisional Publication No. H4-324347 describes a whole-blood analyzing element having a plural number of oxygen-supplying holes so as to increase the oxygen supply. The analyzing element may or may not have a light-transmissive support and has no lightshielding layer. The dye-forming composition comprises 4-aminoantipyrine and its coupler, but the calorimetric measurement is carried out at a wavelength of 541 nm. Therefore, the disturbance by the red color of whole blood is not completely obviated, and the dye-forming reaction (i.e., color-forming reaction) does not proceed rapidly.
Japanese Patent Provisional Publication No. H9-121894 discloses a whole blood-analyzing strip comprising an anisotropic film which contains an acrylic polymer having low reactivity to the hematocrit in whole blood. This strip has a disadvantage in that satisfactory uniform distribution of the spotted whole blood in the lateral direction is not attained.